The pumping action of the heart is controlled by electrical stimulation of myocardial tissue. Stimulation of this tissue in various regions of the heart is controlled by a series of conduction pathways contained within the myocardial tissue.
Cardiac arrhythmias arise when the pattern of the heartbeat is changed by abnormal impulse initiation or conduction in the myocardial tissue. The term tachycardia is used to describe an excessively rapid heartbeat resulting from repetitive stimulation of the heart muscle. Such disturbances often arise from additional conduction pathways which are present within the heart either from a congenital developmental abnormality or an acquired abnormality which changes the structure of the cardiac tissue, such as a myocardial infarction.
One of the ways to treat such disturbances is to identify the conductive pathways and to sever part of this pathway by destroying these cells which make up a portion of the pathway. Traditionally, this has been done by either cutting the pathway surgically; freezing the tissue, thus destroying the cellular membranes; or by heating the cells, thus denaturing the cellular proteins. The resulting destruction of the cells eliminates their electrical conductivity, thus destroying, or ablating, a certain portion of the pathway. By eliminating a portion of the pathway, the pathway may no longer maintain the ability to conduct, and the tachycardia ceases.
One of the most common ways to destroy tissue by heating has been the use of electromagnetic energy. Typically, ablating elements emit radiofrequency (RF), microwave, ultrasound, and/or laser energy to destroy tissue. With RF energy, a catheter with a conductive inner core and a metallic tip are placed in contact with the myocardium and a circuit is completed with a patch placed on the patient's body behind the heart. The catheter is coupled to an RF generator such that application of electrical energy creates localized heating in the tissue adjacent to the distal (emitting) electrode. The peak tissue temperatures during catheter delivered application of RF energy to the myocardium occur close to the endocardial surface, such that the lesion size produced is limited by the thermodynamics of radiant heat spread from the tip. The amount of heating which occurs is dependent on the area of contact between the electrode and the tissue and the impedance between the electrode and the tissue. The higher the impedance, the lower the amount of energy transferred into the tissue. For ultrasonic ablation, the amount of heating may depend on the energy of the signal, the focal length of the transducer delivering the energy, the frequency of the energy, and/or the duration that the energy is applied to the tissue to be ablated.
Power may be delivered to the ablating elements through channels, with one channel per power source. In embodiments with less than one power source per channel, where each channel does not have its own power source, power would have to be switched between channels fast enough so that the rate of tissue ablation decreases insignificantly. The time it takes the rate of tissue ablation to decrease insignificantly is known as the thermal time constant. The switching cycle must be at least as fast as the thermal time constant. One switching cycle is defined as the time it takes to circulate power among all channels for one full cycle. If an AC power source is used, this produces modulated pulse train power signals in the channels. If there were one power source supplying power to M channels with a thermal time constant T, power would have to switch from one channel to the next in T/M seconds.
While a solid state switching matrix can be used to increase switching speed, such a matrix would generate electrical parasitics which would result in unacceptable signal distortion. Signal distortion can be a change in signal power at the operating frequency, and it can be a harmonic distortion (i.e. the ratio of the signal power at a frequency other than the operating frequency to the signal power at the operating frequency). During operation, solid state switches can produce distortions of up to a 15% change in signal power, and/or a total harmonic distortion of up to 5%. However, during calibration of an ablating device, these distortions should be less than approximately 0.25% (for either form of distortion). A need therefore exists for an apparatus that reduces the number of power sources required without generating unacceptable levels of signal distortion.